Expandable polystyrene of low thermal conductivity can be a useful insulation material in building and construction, wherein insulation boards are made from pre-expansion of beads of adequate particle size, block moulding and cutting. However, it can be problematic to directly prepare styrenic polymeric beads of a certain bead size range in a suspension polymerization process that can be used for production of expandable polymers, and poor mean bead size control results in less predictable output of particle sizes that can be used for block moulding, but also shape moulding. Inadequate particle sizes are considered off-grades.
U.S. Pat. No. 4,173,688 describes a process for preparation of expandable styrenic polymer particles of substantially narrow bead size distribution using polystyrene particles of narrow particle size distribution as seeds for seed polymerization with styrene. Such seed polymerization process enables production of expandable styrenic polymer beads of substantially narrow bead size distribution. However, the expanded and moulded styrenic polymer from this process has poor insulation properties.
U.S. Pat. No. 6,130,265 describes a suspension polymerization process for the preparation of polystyrene particles containing an athermanous agent and a foaming agent to reduce heat conductivity. However, a broad particle size distribution and poor mean particle size control result from inadequate suspension stability control.
WO 2010/066331 A1 describes a two-step process for producing expandable styrenic polymer beads using extruded styrenic polymer granulates and styrene monomer as main raw materials in a seed suspension polymerization process. However, the use of non-spherical oval shaped granulates with rough surface appearance as the styrenic polymer seed may possibly give suspension instability problems. This process requires a potentially more expensive extrusion step before the aqueous seed polymerization step in the reactor.
U.S. Pat. No. 5,189,069 describes a suspension polymerization process, wherein at least 70%, based on the total monomer, of the monomers are polymerized in an aqueous suspension initially to a conversion of at least 70% by weight, followed by gradually adding the last at most 30% of the total monomer in a seed polymerization. Incorporation of athermanous agent and the challenges associated therewith are not described.
Control of mean particle size, particle size distribution as well as exothermic reaction and suspension stability remain challenging issues in suspension polymerization of styrene. In particular, the incorporation of an athermanous agent in the formulation can have substantial adverse effects on these important parameters, making the process especially challenging as the use of conventional suspending agents, initiators and additives is impaired due to the substantial change of hydrophobicity of the polymer particles and inhibition of polymerization initiators.
Furthermore, there remains a challenge to produce expandable polystyrene with lower than four per cent content of foaming agent because of problems in terms of inadequate expansion performances and the inability to produce low density products.
There is a need in the art and thus it is the object of the invention to provide a fast, simple, robust and efficient process for the preparation of a solid particulate vinyl aromatic polymer composition with a substantially improved control of mean particle size and particle size distribution, wherein the polymer particles to be produced besides a polymer of vinyl aromatic monomer can beneficially contain athermanous agent, flame retardant and/or foaming agent. Furthermore, it is desirable that the pre-expansion capabilities and specific properties of the resulting foamed product are not adversely affected—even when an athermanous agent is present and/or even at low contents of foaming agent.